We do not recommend adding additional directory password protection for the wp-admin folder/the WP Dashboard for these 2 main reasons.
1. WordPress already has authentication protection to the wp-admin folder/the WP Dashboard. Adding a second login does not really make the wp-admin folder/the WP Dashboard any more secure.
2. Visitors to your website will have problems registering to your site.
If you still want to do this anyway then you would add directory protection in your web host control panel for the wp-admin folder and then combine that code into your BPS wp-admin .htaccess file.